The invention refers to a hydrodynamic brake, generally known as “retarder”, in particular for motor vehicles.
Such retarders contain two impeller wheels of which one stays fixed (the stator) and the other revolves (the rotor) The two impeller wheels together form a torus-shaped working space. This working space is used for the purpose of braking and is to this end filled up with a work medium, for example with oil, whereas in the non-braking operation area no work medium is present.
In this working space in the non-braking area there is present, however, air. This produces an undesirable braking momentum and thereby power dissipation which increases the energy consumption.
Numerous measures have become known with the intention of eliminating this power dissipation. One of these measures consists therein that the rotor and stator are to be made to be slideable relative to one another in an axial direction. In the non-braking operation the rotor and the stator are made to travel away from one another so that the above-mentioned energy dissipation does not occur.
DE 1600 154 A1 describes this kind of a retarder. Hereby, in order to eliminate the so-called ventilation losses in the non-braking operation, the stator is made to travel away from the rotor. Thereby the rotor stays in an axial direction in one and the same place.
In the known forms of execution the rotor is carried on a hollow shaft. The hollow shaft must have bearings. To that end a bearing is located against the casing of the stator. Such a design is elaborate and furthermore occupies a substantial amount of construction space.
Hydrodynamic clutches with impeller wheels which can be moved to one another are also known. An example is U.S. Pat No. 2 359 930, which demonstrates a drive shaft with a pump wheel (impeller) and a driven shaft with a turbine wheel (runner), whereby the pump wheel has its bearing on the outer thread of the drive shaft in such a way that it can be driven towards or away from the turbine wheel. On a front end of the drive shaft there is provided a ring with springs, whereby the springs exert a force onto the pump wheel, axial to the drive shaft in the direction of the drive.
During starting up, that is to say during the rotation acceleration of the drive shaft, a force works on the pump wheel which causes the pump wheel to migrate onto the thread against the spring force in the direction of the turbine wheel. During normal operation it is held in this position, as long, that is, as the loading to be borne by the drive shaft is for the most part held at a constant. As soon, however, as this loading is reduced, or when the drive line moves over into coast drive, that is to say when the output shaft rotates at a higher revolution speed than the drive shaft, the spring force becomes predominant and pushes the pump wheel into a position at a distance away from the turbine so that the flow cycle in the working space between pump wheel and turbine wheel can no longer effectively transfer a revolution momentum from the pump wheel to the turbine wheel.
The illustrated hydrodynamic coupling thus transmits during start-up and in virtually stationary operation a torque from the drive motor to the driven machine via a flow circulation. In the case of a load reduction on the drive side, the pump impeller is stopped by the turbine wheel (as is shown) and the flow circulation is disturbed and the transmission of torque from the drive motor to the driven machine is automatically interrupted. Hydrodynamic braking of the driven machine is not possible with this hydrodynamic coupling because at first there are no means for fixing the pump impeller and the pump impeller is in a remote position relative to the turbine wheel further in a state which makes braking desirable, namely at the time when the driven shaft rotates faster than the driving shaft.